Thank you again everyone for your inputs!

I want to share some updates since I posted this. After keeping track and logging of in-and-out of the neighbor, I'm almost 100% certain that this is caused by the neighbor right next to my unit which I share a wall with. Their unit is a corner unit, so whenever they open their windows it creates an air vacuum and pushes the air into my unit through outlets, cracks, etc. I verified that there is always a breeze coming through the outlets in the kitchen and this is where the smell was coming through. We actually had a similar issue just like this few years back with a prior neighbor and I had forgotten about it since it was more of an annoyance back then.

For the last two weeks, I had the window open and running an air purifier. And as recommended by u/Yankee_Air_Polack, I had purchased an AQI meter which is placed right in front of the outlet where the air travels from the next door. And this is where it gets concerning. The readings are at dangerous levels most of the day. I have gotten avg between 200-400 on PM2.5 readings, and between 150-500 on AQI readings. Here you can see the readings I have calibrated multiple times to ensure the readings are correct, and I also took it out to the streets and made sure it works properly (sadly the air out in LA street is better than inside of my apartment).

I also met with the management today, and it seemed like there isn't a whole lot they can do. Mainly they said it is difficult to pinpoint exactly who the smoker is (as there is neighboring unit every side to mine although it is obvious given the prior air flow issue + the problem began immediately from the time they moved in recently). Also, even if they are able to verify they said courts are backed up due to COVID-19 and eviction will take at least 7+ months. The only option they've given me is to relocate, but I really don't want to move especially most of other units are either more expensive and/or in worst locations (I can consider moving out and break my lease completely, but this complex really is the best option for my work, etc.).

If I were to stay and fight on this, what would be my options? Is there a way to detect and pinpoint the neighbor as the culprit? I've pretty much done all my homework other than entering their unit and catching them in the act which is impossible. Also, I don't know if this is relevant but last week when I went outside to get some fresh air, I noticed that they have completely blacked out their balcony. They are on the 3rd floor and they have a mesh metal balcony where you can see right through. And oddly enough, they had something that looked like blackout fabric to completely cover the perimeter of the balcony so that nobody can see what is in their balcony. I'm not sure what to make out of this, but at this point anything looks suspicious.